Interpretive Summary: We are studying the mechanism by which bacteria are able to cause plant disease. Towards this end, we are analyzing the Pseudomonas syringae pv. syringae (the causative agent of brown spot disease of snap bean - an important field disease in Wisconsin) as a model system. We hope to gain an understanding of the molecular biology of the genes and gene products that are required for the disease process. Ultimately, this understanding will provide information of use to breeders and plant molecular biologists that will enable them to develop plant lines resistant to disease organisms through the alteration of specific plant products that are required by the pathogen. In this study we found that the expression of certain ribosomal proteins can restore the protease function of bacteria which previously had been blocked by mutation. These results provide further clues to the control of essential pathogenic items such as the elaboration of tissue- dissolving enzymes.

Technical Abstract:
The lemA gene of Pseudomonas syringae pv.syringae encodes the sensor-kinase of a bacterial two-component signal system. Phenotypes that are lemA- dependent in P.syringae include lesion formation on bean and production of extracellular protease and the antibiotic syringomycin. The gacA gene has been identified as encoding the response regulator of the lemA regulon. To oidentify additional components that interact with LemA, suppressors of a lemA mutation were sought. A locus was identified that, when present in multiple copies, restores extracellular protease production to a lemA insertional mutant of P.syringae. This locus was found to encode the P.syringae homologs of translation initiation factor IF and ribosomal proteins L20 and L35 of Escherichia coli and other bacteria. Deletion analysis and data from Western blots using anti-IF3 antiserum suggest that protease restoration requires overexpression of either L20 or L35 but not of IF3. It is unclear how alteration of ribosomal protein expression compensates in this instance for loss of a transcriptional activator.